This invention relates to ultrasonic flowmeters and more particularly to such flowmeters employing waveguide antennas to interrogate a moving fluid.
The history of ultrasonic flowmeters is reviewed by the applicant in Chapter 5, pages 407-525 in the book Physical Acoustics--Vol. 14, edited by Mason and Thurston and published by Academic Press (1979). The origins of some of the most common ultrasonic flowmeters in use today, such as the doppler and the contrapropagating transmission (upstream-downstream) methods are traceable to early patents such as those of Chilowsky and Langevin (1923) U.S. Pat. No. 1,471,547 and Rutten (1928-1931) German Pat. No. 520,484, respectively. These and subsequent investigators used two or more transducers, generally piezoelectric types, to launch and detect ultrasonic waves the velocity or frequency of which was modified by interaction with the flowing fluid. Transducer arrays were used by Chilowsky and Langevin and subsequently by Swengel (1956) as described in U.S. Pat. No. 2,746,291. Swengel also used a pair of parallel waveguides to interrogate water in large rectangular ducts over a sheetlike path oriented obliquely to the flow. Raptis et al. in the 1978 IEEE Ultrasonics Symp. Proc., pp. 291-295 utilized the zigzagging SV shear wave (vertically polarized) in a pipe wall to act as an extended source for doppler interrogation of liquid flow. However, this "simulated array" is not classed as a waveguide mode antenna because the pipe wall bounds the wave in the radial direction only, but does not bound the wave in the circumferential direction. For the purposes of this invention, the term waveguide shall mean an elongated structure in which one or more wave modes are substantially confined by means of structural boundaries forming a closed surface surrounding the principal waveguide axis of propagation. The closed surface may have a circular or noncircular cross section and the waveguide itself may be solid or hollow. The ultrasonic wave will usually or generally fill substantially the entire solid portion of the waveguide.
Very few investigators other than Swengel have used waveguides as antennas. Sunthankar (1973) IEEE Trans. Sonics and Ultrasonics SU-20 274-278 used a ribbon-like structure excited into flexural vibration as an ultrasonic radiator. Sunthankar used strain gages to excite the radiator; Swengel used piezoelectric crystals at the undamped end of each of his waveguides. Swengel disclosed transducers attached at opposite ends of parallel waveguides, and waveguides tilted with respect to flow direction to compensate for the oblique launching of waves in the water from the traveling wave in the waveguide.
Prior art buffer rods or waveguides used in ultrasonic flowmeters include the SV mode ultrasonic wave as described by the applicant in U.S. Pat. No. 3,477,278, (1969). In that patent transmission occurred substantially at the beveled ends, leading to fluid interrogation along a solid cylindrical (non-sheet-like) path.
There are several disadvantages with known prior art ultrasonic flowmeters. First of all, the known flowmeters are expensive and usually must operate at temperatures below the transducer's curie point. Additionally, the accuracy of known flowmeters is degraded when measuring the velocity of a multiphase liquid such as a slurry with entrained air, or a fluid which tends to deposit residues on conventional transducers. In Swengel's flowmeter, the transducers for launching or detecting waves were separate from the waveguide antennas themselves giving rise to bonding problems between the transducer and waveguide. Another disadvantage of prior art flowmeters was the need to transpose electrically the launching and detecting transducers to interrogate the fluid in the upstream and downstream directions.
It is an object of this invention, therefore, to provide an ultrasonic flowmeter using waveguide antennas which is inexpensive and reliable and which maintains its accuracy at high temperatures and in the presence of multiphase fluids and liquids which tend to deposit residues on the waveguide antenna.
It is a further object to provide such a flowmeter which substantially eliminates errors resulting from non-axial flow components.
Yet another object is a flowmeter using waveguide antennas for contrapropagating interrogation which does not require transposing the launching and detecting transducers.
Another object is to provide apparatus for measuring a fluid's characteristic impedance or its mass or volumetric flow rate.
Other objects, features and advantages of the invention will be pointed out hereinafter.